Hundreds of detainees revolted earlier in Mayafter five inmates were to be taken to the notorious Sednaya prison near Damascus so that death sentences passed by an extra-judicial military tribunal could be carried out.

Government forces surrounded the prison and fired tear gas during that revolt in an effort to quell the unrest, according to the Observatory.

International and Syrian human rights organisations have decried conditions inside Syrian prisons before and during the war.

Between March 2011 and the end of 2015, the Syrian Network for Human Rights documented the arrest and detention of more than 117,000 people.

More than 60,000 people have been killed through torture or died in dire humanitarian conditions inside regime prisons throughout the country's five-year uprising, according to a report by the Observatory.

In a December 2015 report, Human Rights Watch (HRW) said its researchers had found "evidence of widespread torture, starvation, beatings, and disease" in government jails and detention centres.